I have always been able to include information that you would include in a Brand Standards plan into the business plan itself. I don't see the need of creating the "same" document twice if you can fit into one.
However, I would say that this also depends on on the size of your company. If you have quite a lot of people working for you (50+ or so) then it just might be wise to create this plan seperate simply for organizational purposes. I say this because I know that if a big group of people come together to address one issue on a document, tons of issues with be addressed and everything would just HAVE to be changed. /sarcasm